The present disclosure relates generally to hybrid or gasoline vehicles, and particularly to systems and methods for managing emissions.
There is interest in producing passenger vehicles with minimal evaporative emissions (e.g., hydrocarbon emissions from a hydrocarbon-based fuel). Evaporative emissions (evap) from vehicles are subject to regulatory requirements that set limits and require on-board diagnostics to verify that a vehicle's emissions control system functions as designed and tested during vehicle certification.
Evaporative emissions control can be an issue for any vehicle that includes on-board fuel storage. For example, even though not primarily powered by conventional fuel, hybrid electric vehicles may require evaporative emissions control. An example of a hybrid electric vehicle is one that is powered by an electric motor having a rechargeable battery (e.g., a lithium-ion battery) and an alternative power source, such as an internal combustion (IC) engine (e.g., using gasoline or diesel fuel). The operating range and power of a battery-powered electric vehicle can be increased using an on-board electric generator driven upon demand by the IC engine. For relatively short driving excursions (e.g., under 50 miles), the capacity of the battery is sufficient and the IC engine is not required to be run. At the completion of such short excursions, the battery is recharged, for instance, by “plugging in” the vehicle to a shore based power source, such as conventional AC electric power provided by a public utility. Such a vehicle is sometimes called a plug-in hybrid vehicle (PHEV) or extended range electric vehicle (EREV).
The IC engine typically needs to operate in order for a typical hybrid vehicle to operate for longer distances (e.g., a few hundred miles). As a result, despite the IC engine's intermittent usage, the IC engine will, of course, require on-board fuel storage. The engine's fuel (e.g., gasoline) is stored in a vehicle fuel tank and is exposed to ambient heating, which increases the vapor pressure of the volatile hydrocarbon fuel. In conventional IC engines, fuel tank vapors (emissions), which typically comprises lower molecular weight hydrocarbons, are vented to an evaporative emissions control canister (or “evap canister”) containing high surface area carbon granules for temporary adsorption of the fuel tank emissions. Later, during operation of the IC engine, ambient air is drawn through the carbon granule bed to purge adsorbed fuel from the surfaces of the carbon granules and carry the removed fuel into the air induction system of the IC engine. However, because hybrid vehicles may be used primarily for short range or local trips, the IC engine may not run for several days. As a result, no purging (cleaning) of the evaporative emission control canister occurs. If the evaporative emissions control canister is not purged, diurnal vapors can escape through the canister into the atmosphere as breakthrough diurnal emissions. An example of a fuel tank and canister system for purging such vapors is described in U.S. Pat. No. 7,448,367, which is herein incorporated by reference in its entirety, and is shown in FIG. 1.
In this exemplary system 0, a fuel inlet 1 is provided for delivering fuel to the fuel tank 2. A fuel tank pressure sensor 6 is mounted in the fuel tank 2 to monitor pressure within the fuel tank 2. The sensor 6 is coupled to a vehicle controller that monitors the pressure of the fuel tank 2. Vapor escapes from the fuel tank 2 through a vapor outlet 3 and into a first inlet 5A of the evap canister 4. A valve 8 is positioned at a second inlet 5B of the evap canister 4 that allows introduction of air into the evap canister 4 to purge the vapor out through an outlet 5C and drive the vapor to the combustion chamber of the IC engine. A purge valve 7 (normally closed) can open and close to let the purged vapor exit the evap canister 4. A pump 9 can be provided that drives the air into the evap canister 4 to check for leaks (e.g., when the IC engine is off).
Problems with a system such as shown in FIG. 1 include the following: too many sealed components (e.g., canister, purge valve, etc.) resulting in durability issues and potentially higher cost than necessary because only the fuel tank 2 actually needs to be sealed to prevent diurnal vapor generation; the evap canister 4 should be sealed only as necessary to prevent thermal bleed emissions between loading and purging the evap canister 4; too many possible leak paths and possible leak detection failures; purging the evap canister 4 also purges the fuel tank 2, which results in undesirable canister 4 loading and fuel weathering); performing a rationality check of the pressure sensor 6 is difficult because tank pressure has to be released which results in undesirable canister 4 loading and fuel weathering.
A sealed fuel tank 2 may generate little diurnal vapors. However, the fuel tank 2 will experience several psi pressure/vacuum changes due to diurnal temperature changes. The evap canister 4 is used only for capturing refueling vapors, which will be purged and consumed only when the tank fuel is consumed by the IC engine. Although the fuel tank 2 is sealed to prevent diurnal vapor generation, the evap canister 4 is also sealed to prevent thermal bleed emissions. In a particular scenario, the evap canister 4 is loaded (to or near capacity) with refueling vapor and then experiences several days/weeks of diurnal temperature cycles. When a loaded evap canister 4 is subjected to diurnal temperature increase, some air and hydrocarbons will be expelled from the evap canister 4 due to thermal expansion and hydrocarbon desorption from the carbon granules in the evap canister 4. To limit the thermal bleed emissions, the evap canister 4 is also sealed along with the fuel tank 2, as shown in FIG. 1. However, sealing both components produces some problems including, for example, the possibility of leaks in the evap canister 4 and valves (e.g., due to pressure/vacuum cycling fatigue), difficulty of purging the evap canister 4 without venting the fuel tank 2.